At present, magnetic recording reproduction devices are being designed to have higher recording density in order to achieve a large capacity with a small size. In the field of a hard disc drive, which is a typical magnetic storage device, a device having an areal recording density of more than 10 Gbits/in2 is commercialized already, and such a rapid progress in the technology can be observed that even the practical use of a device with 20 Gbits/in2 is discussed.
As the technical background for enabling such high recording density, significant factors are the improvement in the performance of a medium and in the performance of a head-disc interface as well as the improvement in the linear recording density achieved by the appearance of a new signal processing mode such as a partial response. Here, the partial response is a mode of intentionally providing a known intersymbol interference at the time of waveform equalization conducted for avoiding an intersymbol interference when the linear recording density is increased. This mode is characterized in that the deterioration of a bit error rate can be prevented compared to a conventional peak detection or an integral detection.
In recent years, however, in addition to the appearance of such a signal processing mode, the main factor for improving the surface recording density is that the tendency toward an increase in the track density is significantly exceeding the tendency toward an increase in the linear recording density. This is due to the fact that a magneto-resistive type head, which has exceedingly excellent reproduction output performance compared to a conventional inductive type magnetic head, has come into practical use. At present, due to the commercialization of the magneto-resistive type head, a signal with a track width as small as several μm can be reproduced at an excellent S/N ratio. On the other hand, along with a further improvement in the performance of the head in the years to come, a track pitch is expected to reach the submicron range in the near future.
Now, for a magnetic head to scan such narrow tracks accurately and to reproduce signals at an excellent S/N ratio, the tracking servo technology of the head plays an important role. As for such tracking servo technology of present hard disc drives, recording tracks are formed in circular manner on a hard disc. And within a revolution of the disc, that is, within an angle of 360 degrees, a single region called a wedge is repeatedly provided at a constant interval, where a servo signal for tracking, an address information signal and a reproduction clock signal etc. are recorded. Hereinafter, the servo signal for tracking, the address information signal and the reproduction clock signal etc. are referred to as preformat signals, and the process of recording these signals in advance is referred to as a preformat recording. The magnetic head reproduces these signals at a constant interval so as to scan on the tracks accurately while identifying and correcting the position of the head.
The above-mentioned preformat signals such as the servo signal for tracking, the address information signal and the reproduction clock signal serve as reference signals for the magnetic head to scan accurately on the tracks. Therefore, when these signals are recorded, positioning is required to be performed correctly with precision. In a present hard disc drive, after a disc is incorporated into the drive, by using a single-purpose servo recording device called a servo track writer, a preformat recording is performed by strictly controlling the position of a magnetic head.
The preformat recording of the signals such as the servo signal for tracking, the address information signal and the reproduction clock signal by the magnetic head with the use of the single-purpose servo track writer as mentioned above has the following problems.
First, a recording by a magnetic head basically is a linear recording based on the relative movement of a head and a medium. Therefore, the above-mentioned method for recording by strictly controlling the position of the magnetic head with the single-purpose servo track writer requires a great amount of time for the preformat recording. In addition, since the single-purpose servo track writer is quite expensive, the cost for the preformat recording becomes extremely high.
Secondly, due to a spacing between the head and the medium, and due to a broadening of a recording magnetic field caused by the pole shape of the recording head, the magnetic transition lacks in sharpness in the track edge portion of the recorded preformat signals. In the present tracking servo technology, the position of a head is detected by the change in the reproduction output amplitude at the time when the head went off-track and scanned. Therefore, with regard to the signal tracks where the preformat recording was performed, the reproduced signal is not only required to have an excellent S/N ratio when scanning accurately on the tracks just as when data signals recorded between servo areas are reproduced, but also to have a steep change in the reproduction output amplitude at the time when the head went off-track and scanned, that is, sharp off-track characteristics. The above problem goes against this requirement, which makes it difficult to provide the accurate tracking servo technology for recording of submicron tracks in the years to come.
Now, as means to solve the problems in the preformat recording by the magnetic head as mentioned above, the preformat recording technology proposed by the present inventors in JP10(1998)-40544A mainly is the technology of using a master information carrier including a base on which a pattern of ferromagnetic thin films corresponding to information signals is formed, and bringing the surface of the master information carrier into contact with the surface of a magnetic recording medium so as to perform a surface transcription recording as a whole of a magnetization reversal pattern corresponding to the pattern of the ferromagnetic thin films formed on the surface of the master information carrier for a magnetic recording medium.
According to the configuration disclosed by the same publication, the magnetization reversal pattern corresponding to the pattern of the ferromagnetic thin films in the master information carrier is transcribed and recorded in the lump on the magnetic recording medium by a recording magnetic field arising from the ferromagnetic thin films formed on the surface of the master information carrier, which is magnetized in one direction. In other words, by forming patterns of the ferromagnetic thin films corresponding to the servo signal for tracking, the address information signal and the reproduction clock signal etc. on the surface of the master information carrier by the photolithographic technique etc., the information signals corresponding thereto can be recorded in the magnetic recording medium for the preformat recording.
While a recording by a conventional magnetic head basically is a dynamic linear recording based on the relative movement of a head and a medium, the above-mentioned configuration is characterized in that this recording is a static areal recording without accompanying relative movement of the master information carrier and the medium. Due to such a feature, the technology disclosed in JP10(1998)-40544A can be extremely effective for solving the above-mentioned problems related to the preformat recording, as will be explained below.
First, due to the feature of surface recording, the time required for the preformat recording is much shorter than that for recording by the conventional head. Moreover, the expensive servo track writer for recording while strictly controlling the position of the magnetic head is not required. Therefore, the productivity related to the preformat recording can be improved significantly, and at the same time, the production cost can be reduced.
Secondly, due to the feature of static recording without accompanying relative movement of the master information carrier and the medium, a spacing between the surface of the master information carrier and the surface of the magnetic recording medium at the time of recording can be reduced to a minimum by securely contacting them to each other. Moreover, the broadening of a recording magnetic field due to the pole shape of the recording head does not occur as in the case with the recording by a magnetic head. Therefore, the magnetic transition at the track edge portion where the preformat recording was performed has excellent sharpness compared with the recording by a conventional magnetic head, and more accurate tracking is possible.
One example of the surface configuration of a conventional master information carrier disclosed in the same publication is shown in FIG. 21. FIG. 21 shows a disc-shaped master information carrier configured so as to perform a lump sum recording of preformat signals for a magnetic recording medium such as a hard disc, and wedge pattern areas 74 are formed in one circle of a disc, that is, over an angle of 360 degrees at a constant interval, with pattern shapes of ferromagnetic thin films corresponding to preformat signals such as a servo signal for tracking, an address information signal and a reproduction clock. In addition, 75 shows areas between wedges, and these areas correspond to data areas on a magnetic recording medium. Also, 76 is a marker used for positioning the magnetic recording medium at the time when it is closely contacted with the surface of the master information carrier.
An enlarged view of a portion A of the master information carrier shown in FIG. 21 is shown in FIG. 22. FIG. 22 shows the pattern configuration of ferromagnetic thin films 63 formed according to the preformat signals inside the wedge, which is provided at a constant angle in the circumferential direction of the magnetic recording medium, only for 10 tracks in the radial direction of a master information carrier 62 (that is, in the direction of recording track width). Also for reference, in the area between wedges 75, track portions where data will be recorded on the magnetic recording medium after the preformat signals are recorded in the magnetic recording medium are shown by broken lines. On the surface of an actual master information carrier, patterns of the ferromagnetic thin films as shown in FIG. 22 are formed, according to the recording area of the magnetic recording medium where the preformat signals are recorded, at a constant angle in the circumferential direction and also for all the recording tracks in the radial direction of the disc.
The preformat signals are formed as arrays of the ferromagnetic thin films 63 included, for example as shown in FIG. 22, in the areas of the clock signal, the servo signal for tracking, the address information signal etc., and each area is arranged sequentially in the longitudinal direction of the track on the surface of the master information carrier 62. The hatched portions in FIG. 22 are the ferromagnetic thin films 63. In addition, the planar shapes of the ferromagnetic thin films 63 in FIG. 22 are all rectangular, but in fact the shapes are not limited thereto and can be formed into different shapes according to the embodiment.
FIG. 23 and FIG. 24 show examples of cross-sectional configurations of the master information carrier of FIG. 22 taken on alternate long and short dash line LL′. The alternate long and short dash line LL′ corresponds to the circumferential direction of the magnetic recording medium, and the lateral direction of the surface also matches the time base direction of the signal at the time when the signal recorded in the magnetic recording medium is reproduced by a magnetic head. The magnetic information carrier 62 may be configured such that pattern shapes made of the ferromagnetic thin films 63 are buried and arranged in a surface portion of a nonmagnetic base 64 as shown in FIG. 23, or such that pattern shapes made of the ferromagnetic thin films 63 are arranged in the form of protrusions on the surface of the nonmagnetic base 64 as shown in FIG. 24. However, in view of durability or long-life of the master information carrier, the configuration shown in FIG. 23 is superior.
In the above-mentioned conventional preformat technology, the patterns of the ferromagnetic thin films on the surface of the master information carrier correspond to the magnetized pattern to be recorded in the magnetic recording medium. Therefore, the patterns of the ferromagnetic thin films should be arranged, for example, in and on the surfaces of the master information carriers illustrated in FIG. 23 and FIG. 24 such that a pattern length A of each ferromagnetic thin film or a distance B between the individual patterns of the ferromagnetic thin films corresponds to a desired signal length in a magnetized pattern to be recorded in a magnetic recording medium, that is, to a length between a pair of magnetic transition areas adjacent to each other in the magnetized pattern.
However, according to the examinations conducted by the present inventors, the length between the magnetic transition areas in the magnetized pattern recorded on the magnetic recording medium in fact does not accurately match the length A of each pattern of the ferromagnetic thin films and the distance B between the individual patterns of the ferromagnetic thin films Therefore, when the pattern length A of the ferromagnetic thin films or the distance B between the patterns of the ferromagnetic thin films is set so as to match the length between the magnetic transition areas desired on the magnetic recording medium accurately, the length between the magnetic transition areas actually recorded on the magnetic recording medium differs from the desired length. As a result, with regard to a reproduction waveform at the time when the recorded magnetized pattern is reproduced by a magnetic head, the position of the reproduction pulse is shifted from the desired pulse position by a certain time.
In this case, it would not be a problem when the shift quantity of the reproduction pulse mentioned above is sufficiently small in ratio to a detection window width of a reproduction signal processing circuit. However, when the amount exceeds the permissible limit of the detection window width, the reproduction signal processing circuit cannot detect the reproduction pulse, so that a reproduction signal error arises.
Furthermore, the preformat recording using the master disc disclosed in the same publication is effective not only for a conventional in-plane magnetic recording medium but also for a vertical magnetic recording medium to be used for performing super high density recording in the future, and the development of its application is awaited.
The magnetic recording method for recording in a vertical magnetic recording medium disclosed in the same publication will be explained below. FIG. 25A to FIG. 25C are cross-sectional views of a vertical magnetic recording medium 61 taken in the circumferential direction of a disc, and the lateral direction of the surface matches the time base direction at the time when a magnetized pattern to be recorded in the vertical magnetic recording medium 61 is reproduced by a magnetic head.
First, as shown in FIG. 25A, the vertical magnetic recording medium 61 where preformat signals are recorded is prepared. Next, as shown in FIG. 25B, the surface of the master information carrier 62 is contacted closely with the surface of the vertical magnetic recording medium 61, and an external magnetic field 65 is applied in the direction perpendicular to the surface of this master information carrier 62 In addition, the example shown in FIG. 25B uses the master information carrier according to the configuration of FIG. 23, but the master information carrier according to the configuration of FIG. 24 may be used as well.
By applying the magnetic field 65, leakage flux 66 corresponding to the shape pattern of the ferromagnetic thin films 63 is generated on the surface of the master information carrier 62. Accordingly, recorded magnetization 67 with a pattern corresponding to the shape pattern of the ferromagnetic thin films 63 is formed in the vertical magnetic recording medium 61 as shown in FIG. 25C.
As a result, as shown in FIG. 25C, in the vertical magnetic recording medium 61, a magnetized pattern, which includes a non-recorded area corresponding to a portion between the ferromagnetic thin films on the surface of the master information carrier and an area where magnetization was recorded by the leakage flux from the ferromagnetic thin films, is formed in which the two areas are arranged alternately via a magnetic transition area 68. In addition, the recorded magnetized pattern of FIG. 25C shows a case in which the vertical magnetic recording medium 61 is erased to a neutral point in advance by applying a magnetic field alternately in opposite directions or through application of a thermomagnetization method or the like prior to the recording by using the master information carrier 62.
On the other hand, as shown in FIG. 26A, prior to the operation of closely contacting the vertical magnetic recording medium 61 with the surface of the master information carrier 62, by applying a d.c. magnetic field to the vertical magnetic recording medium 61 and providing initial magnetization 69, as shown in FIG. 26C, it is possible to form the recorded magnetization 67 with a pattern including the residual magnetization of the initial magnetization 69 and the magnetization recorded by the leakage flux 66, arranged alternately via the magnetic transition area 68. In this case, the polarity of the initial magnetization 69 should be opposite to the polarity of the applied magnetic field 65. When the magnetized pattern recorded in the vertical magnetic recording medium 61 is reproduced by using a magnetic head, about twice as large a reproduction signal amplitude can be obtained from the recorded magnetized pattern shown in FIG. 26C as that from the recorded magnetized pattern shown in FIG. 25C, so that this configuration is more preferable.
In this way, as shown in FIG. 25B and FIG. 26B, when a preformat recording is performed for a vertical magnetic recording medium, by applying the magnetic field 65 in the direction perpendicular to the surface of the master information carrier 62, the ferromagnetic thin films 63 are magnetized in the vertical direction of the film surface, that is, in the thickness direction. However, it became clear that sufficient vertical recording performance cannot necessarily be obtained by such a method.
This principle will be explained by referring to FIG. 27. 80 shows a vertical magnetic field distribution in the vicinity of the master information carrier surface obtained by the leakage flux 66 from the applied magnetic field 65 and the ferromagnetic thin films 63. In order to obtain excellent recording performance by the magnetic recording method disclosed in the same publication, by focusing magnetic flux on the ferromagnetic thin film portions having high magnetic permeability inside the master information carrier, the magnetic field at the portion between the individual ferromagnetic thin films needs to be reduced sufficiently compared to the volume of the applied magnetic field (the level shown by a straight line 81) and the magnetic field at the ferromagnetic thin film surface needs to be increased sufficiently compared to the volume of the applied magnetic field (the level shown by the straight line 81).
However, as a result of further examinations conducted by the present inventors, it became clear that it is difficult to obtain the preferable vertical magnetic field distribution as described above by the method disclosed in the same publication. In other words, a demagnetizing field in the direction perpendicular to the film surface is strong inside the ferromagnetic thin films 63, so that sufficiently large leakage magnetic flux 66 contributing to the recording, that is, a magnetic field in the vertical direction, cannot be obtained at the surface of the ferromagnetic thin films 63.
Furthermore, since the magnetic flux is not focused sufficiently on the ferromagnetic thin films 63 inside the master information carrier, a strong vertical magnetic field exceeding the half of the applied magnetic field 65 is generated also at the portion between the ferromagnetic thin films 63. As a result, the recorded magnetization 67 in the vertical magnetic recording medium 61 becomes much smaller than the value of the residual magnetization intrinsic in the vertical magnetic recording medium 61, and the amplitude of a reproduction waveform 82 reproduced from this recorded magnetized pattern (See FIG. 27) becomes much smaller than a reproduction signal amplitude 83 reproduced from the magnetized pattern recorded by a conventional magnetic head.
This problem can be solved to some degree by increasing the thickness of the ferromagnetic thin films 63 on the master information carrier 62 and reducing the demagnetizing field inside the ferromagnetic thin films. However, in order to obtain sufficient improvement by the above-mentioned method, it is necessary to increase the thickness relative to the pattern length of the ferromagnetic thin films shown in the cross-sectional view of FIG. 27 at least as much as twice to three times. To form a shape pattern of ferromagnetic thin films having such a high aspect rate is extremely difficult from the viewpoint of the lithographic technique used in the manufacturing process of a master information carrier.
In order to obtain a larger reproduction signal amplitude with the conventional method disclosed in the same publication, as shown in FIG. 26A, it is necessary to erase the magnetization in the vertical magnetic recording medium 61 by applying a d.c. magnetic field prior to the operation of closely contacting the vertical magnetic recording medium 61 with the surface of the master information carrier 62, and to provide the initial magnetization 69 for the vertical magnetic recording medium 61.
However, a vertical magnetic recording medium has a strong demagnetizing field in the direction perpendicular to the film surface of a magnetic layer, so that it is difficult to realize the d.c. erasing state uniformly and stably in a large area over the entire disc surface. That is, due to the demagnetizing field at the time when the magnetization is erased by applying a d.c. magnetic field, the initial magnetization 69 in the vertical magnetic recording medium 61 becomes extremely small compared to the value of the residual magnetization intrinsic in the vertical magnetic recording medium 61.
Furthermore, in the course of time after the application of a d.c. magnetic field, the magnetic domains in which the magnetization is locally reversed by the demagnetizing field increase, so that the initial magnetization 69 is further demagnetized. Therefore, it is difficult to obtain a uniform and sufficiently large reproduction signal amplitude over the entire disc surface by the conventional method described above.
In view of the foregoing problems, it is a first object of the present invention to solve the above-mentioned problems in the preformat technology disclosed in JP10(1998)10-40544A and to provide a master information carrier capable of recording preformat signals causing no reproduction signal errors in an in-plane magnetic recording medium by approximating a length between magnetic transition areas of a magnetized pattern to be recorded in an in-plane magnetic recording medium by a preformat recording to a more desirable set point, and also to provide an in-plane magnetic recording medium capable of accurate servo tracking by using this master information carrier. Furthermore, a second object is to provide a master information carrier capable of recording preformat signals in a vertical magnetic recording medium exhibiting a uniform and sufficient large reproduction signal amplitude over the entire disc surface, and also to provide a vertical magnetic recording medium capable of accurate servo tracking by using this master information carrier.